Hydrocarbons, and in particular petroleum, are produced from the ground as a mixture. This mixture is converted to useful products through separation and processing of the streams in reactors. Conversion of the hydrocarbon streams to useful products is often through a catalytic process in a reactor. The catalysts can be solid or liquid, and can comprise catalytic materials on a support. In particular, catalytic materials on a solid support are extensively used. Example catalytic materials include platinum group metals and other metals. During the processing of the hydrocarbons, the catalysts deactivate over time. One primary cause of deactivation is the formation and buildup of coke on the catalyst. The accumulation of coke blocks access to catalytic sites on the catalyst, and access to pores within the catalyst. Regeneration of the catalyst is normally performed through the removal of the coke, where the coke is combusted at a high temperature with a gas including oxygen. These processes can be performed either in a continuous manner, with the catalyst cycled through the reactor and the regenerator, or the process can be performed in a semi-continuous manner, such as with multiple fixed beds, where one bed is taken off stream to regenerate the catalyst, while the other beds continue operation.
With the continuous regeneration process, a recycle gas is continuously passed to a combustion zone in the regenerator, and a flue gas containing the combustion process is removed. The combustion process is controlled through the oxygen content in the recycle gas. The recycle gas stream includes a portion of the flue gas, and an additional stream of new combustion gas, while venting another portion of the flue gas from the regenerator. This helps maintain the temperature of the combustion gas, as well as setting up a steady state condition of continuous addition of spent catalyst and combustion gas to the regenerator, while continuously drawing regenerated catalyst and flue gas. Example catalyst regeneration methods are disclosed in U.S. Pat. No. 5,053,371 to Williamson.
The combustion process can be damaging to the catalyst, and thus improved methods of controlling the combustion process are useful for improving the life of the catalyst in a reactor-regenerator cycle. For example, conventional regenerators operate within particular temperature limits. These limits typically are managed by operating under oxygen limitation in an upper part of the regenerator, such that the maximum temperature that can be obtained is bounded by a concentration of oxygen and temperature of the inlet gas. However, this also restricts the driving force for coke removal within the lower portions of the regenerator.